The present invention relates to absorbent particles and the use of such particles in absorbent articles such as disposable diapers and sanitary pads.
Generally, absorbent articles are designed to have at least three distinct layers:
(1) a liquid pervious topsheet of bonded fibers usually referred to as a non-woven or an apertured film; PA1 (2) an absorbent core containing mainly hydrophilic fibers such as a loose mat of pulp fibers usually referred to as "fluff" often contributing as much as 90% of the overall thickness of the article; and PA1 (3) a liquid barrier outer film which is usually a polyolefin at a thickness of about 0.5-1.5 mil.
Fluids passing through the topsheet are distributed by the pulp fibers and are held within the interstices of the pulp web. Increases in pulp content generally lead to increases in the absorbent capacity of these devices. In light of the relatively low cost of pulp, absorbent articles are often designed with thick, bulky cores to provide high capacity in the absorbent article. The inherent disadvantages of such bulkiness are lack of comfort, visibility through clothing, and the inability of the article to conform to the shape of the body leading to unpredictable shifting and even fluid leakage. Importantly, these bulky items require precious storage space in distribution and in ultimate usage.
The development of superabsorbent polymers (SAP) which absorb at least 3-4 times as much fluid as pulp, has lead to the successful design of much thinner products in which a substantial portion of the fluff has been replaced by SAP granules. While the consumer has accepted and even preferred these thinner articles, certain performance attributes are still not satisfactory and improvements are sought. For example, leakage is still one of the most important and the most critical deficiency remaining in disposable diaper design.
One approach taken to solve this problem has been to incorporate yet more SAP into the product. It is not uncommon in ultra-thin diapers to use as much as 6-15 grams of SAP granules per diaper accounting for up to 60% by weight of the absorbent core in order to achieve significant reductions in bulk or thickness and still provide needed performance.
SAP, generally, are polymeric materials containing water insoluble long chain molecules with a low degree cross-linking which are capable of forming hydrogel networks. In the presence of water or aqueous solutions such as body fluid, these hydrogel networks swell into a soft, resilient "jelly-like" material. When the swelling fluid is 0.9% saline, urine, or synthetic urine, these polymers may ultimately swell up to about 25-40 times their original weight. On the other hand, pulp fibers have a capacity to swell by a factor of only about 7-10 times by comparison.
The SAP materials are typically produced as granules which may then be mixed with pulp fibers during the formation of the absorbent core. Thus, with such highly absorbent granular material, it becomes possible to design and produce absorbent articles with roughly 1/2 to 1/3 of the bulkiness of the 100% pulp core. Reduction in volume of this nature is the subject of numerous U.S. Patents including for example, U.S. Pat. Nos. 4,950,264 (Osborn); 4,467,012 (Pederson); and 4,217,901 (Bradstreet), all incorporated in their entirety by reference herein.
There is unfortunately a disadvantage associated with this improvement. For while SAP is about three-fold more absorbent than pulp, its cost is about four-fold higher. It is not surprising, therefore, that considerable effort has been dedicated toward maximizing or optimizing the cost effectiveness of the superabsorbent. These efforts are the subject of numerous U.S. patents. For example, particle size, modules, degree of neutralization, and residual monomers, are discussed in U.S. Pat. Nos. Re. 32,649 (Brandt) and 5,061,259 (Goldman et al.), both incorporated in their entirety by reference herein.
Furthermore, technical contributions were reported at the Advances in Superabsorbent Polymers Symposium, Fall Meeting 1993 of the American Chemical Society, as published in the Proceedings of the Division of Polymeric Materials: Science and Engineering (Masuda, p. 464; Nagorski, p. 560), incorporated herein by reference.
Prior to the trend toward thinner diapers, SAP was designed initially for maximum capacity and later with increased cross-linking for improved gel stability and absorbency under a load. With the trend toward thinner construction, the speed of liquid acquisition and distribution are also important properties. Accordingly, while SAP particles usually swell to capacity after about one hour exposure to fluid, the uptake rate during the first ten to twenty minutes is now considered critical.
It has been recognized that the rate of absorption could be increased through higher cross-link density of the SAP since the resulting increase in gel strength helps to maintain particle identity during swelling thus reducing particle coalescence. The effect of the latter phenomenon known as "gel blocking" is to block the open spaces in the web, causing a decrease in the rate of absorption. The undesirable drawback with increased cross-linking is the associated reduction in fluid capacity. In U.S. Pat. Nos. 4,587,308 (Makita) and 4,507,438 (Obayashi) (both incorporated in their entirety by reference herein), particles are subjected to cross-linking on the particle surface thereby increasing surface gel strength without compromising the swell capacity within the particle.
In U.S. Pat. No. 3,932,322 (Duchane) (incorporated in its entirety by reference herein), the tendency for particle agglomeration is reduced by admixing a small amount of very fine inorganic oxide particles which tend to coat the SAP particles.
The problem of reduced rate due to blocking of fluid is also addressed in U.S. Pat. No. 5,147,343 (Kellenberger) (incorporated in its entirety by reference herein), wherein the size of the superabsorbent particle is selected to be larger than the pore size of the absorbent core (i.e., at least about 100 microns). This design feature is claimed to provide improved absorbency under a load or under the weight of the user's body.
The importance of particle size is further taught in U.S. Pat. No. 5,180,622 (Berg et al.) (incorporated in its entirety by reference herein). This patent, more specifically, discloses that surface area of the particles controls the rate of fluid uptake. Since small particles have the more favorable ratio of surface area to mass, theory would predict a higher absorption rate with relatively small particles, i.e., about 50 .mu.m. In fact, due to ease of packing, particles this size tend to form a mass of coagulated gel and fluid and flow is impeded by their "gel blocking." In the Berg et al. patent, the trade-off between fluid uptake and gel coagulation is resolved through a process which chemically links small particles into a larger cluster or agglomerate. These new particles have significantly higher swell rates than the precursor particles based on the high surface to mass ratio; however, this benefit is offset to some degree by increased processing cost.
Another approach to solving the trade-off problem is to simply use larger quantities of larger particles; the drawback here is that increased volumes of SAP create additional expense for the manufacturer. Further, the greater fluid capacity realized with more SAP is usually not utilized.
It is also desirable to provide a simple and new means by which average particle size and particle size distribution may be controlled.